The present invention relates generally to waveguide reflectors and, more specifically, to the design of particular types of distributed Bragg reflectors such as may be used in a free electron laser.
Free electron lasers typically operate at wavelengths in the infrared or visible bands and utilize mirrors at the opposite ends of their resonating cavities to direct the radiation field along the optical axis of the cavity. An axially directed electron beam is brought into and out of the ends of the cavity by the use of bending magnets. A typical example of a free electron laser (FEL) is illustrated in U.S. Pat. No. 4,438,513 issued in the name of Luis Elias and assigned to the United States of America.
In an FEL operated in the microwave to far-infrared bands, however, a high-Q resonator is usually required to confine the mode. The resonator must also allow passage of an electron beam. The expanding Gaussian-mode patterns of a conventional confocal or a concentric resonator, however, do not easily fit within the bore of the wiggler-magnet array, especially at frequencies below 100 GHz. As a result, the radiation fields of such conventional resonators must be confined by some means along the 30-100 cm interaction length. Since future FELs must be capable of generating very high average power, the resonators therein will have to be overmoded, i.e., use quasi-optical propagation with cross sections larger than one-half wavelength to avoid thermal damage to the resonator structure.
Reflectors utilizing square-wave corrugations operating with the known principle of Bragg scattering relating to constructive interference at certain angles, called Bragg angles, have been previously proposed for use in waveguides. See, e.g., "Waveguide Resonators with Distributed Bragg Reflectors" by R. Kowarschik and A. Zimmerman, Optica Acta 1982, Vol. 29, No. 4, pages 455-462. According to the prior art literature, however, the shape of the corrugations has little effect on the reflection coefficient. See, e.g., articles by: Marcuse, IEEE Journal of Quantum Electronics, Vol. QE8, pages 661-669, July, 1972; Miles and Grow, IEEE Journal of Quantum Electronics, Vol. QE14, No. 4, pages 275-282, April, 1978; and Yariv et al., IEEE Journal of Quantum Electronics, Vol. QE13, pages 233-251, April, 1977.
Couplers using diffraction gratings built directly into a dielectric waveguide for use in input or output coupling from the waveguide modes to free-space, or substrate propagating modes, are discussed by Yariv et al., IEEE Journal of Quantum Electronics, Vol. QE13, pages 233-251, April, 1977 at pp. 249-251. The waveguide grating couplers discussed by Yariv et al. are built not into metallic waveguides but into dielectric waveguides, and their function is to scatter power out of the waveguide rather than coherently reflecting it inside the waveguide. Also, the corrugations are not blazed. The discussion includes an analysis of the coupling loss of such a waveguide grating with sawtooth-type triangular corrugations.
Bratman et al., in "FELS with Bragg Reflection Resonators Cyclotron Autoresonance Masers Versus Ubitrons", IEEE Journal of Quantum Electronics, Vol. QE19, No. 3, pages 282-295, March, 1983, discuss the applications of sinusoidal corrugated reflectors in FELS, Ubitrons and Cyclotron Autoresonance Masers (CARM).
The sawtooth type triangular corrugations shown in FIG. 35 of the Yariv article and the sinusoidal corrugations of Bratman et al. are not spaced apart with an intervening base.
It is known in the field of optics, particularly as related to diffraction gratings, that "blazing" of grooves of a grating will cause it to be particularly reflective of light at a certain wavelength. See, e.g., Fundamentals of Optics, Jenkins and White, McGraw-Hill, 1957. As defined in the McGraw-Hill Dictionary of Scientific and Technical Terms, 3rd Edition, a "blaze-of-grating technique" is an optics technique whereby ruled grooves of a diffraction grating are given a controlled shape such that they reflect as much as 80% of the incoming light into one particular order for a given wavelength. Such techniques are also effective in the microwave region where blazed corrugated reflectors may be used to advantage in the design of high-Q FEL resonators.
A certain amount of the development of the microwave devices mentioned above involves, at least to some degree, an empirical approach to the realization of optimal structures. One particular high-Q FEL resonator design has been found in practice to develop an exceedingly high Q which achieved a large coupling coefficient for a relatively short reflector which was not explainable by existing theoretical arguments. Other designs exist which give comparable results but violate both the theoretical and blaze design criteria. Coupled mode theory (CMT) analysis may suggest a potential capability of calculating reflectivity of a given corrugation when higher-order corrections are included but such theory is not yet suitable to use to generate the ideal surface of a given mode reflector.
Empirical progress in reflector design suggests that better reflectors can eventually be designed and tailored to specialized applications, but the present empirical and analytical procedures are not up to the task of identifying the critical conditions or providing the design criteria. One purpose of this disclosure, therefore, is to demonstrate an independent approach to designing a resonator to realize a optimum resonance of a selected mode, as an alternative to the conventional approach of determining what modes will be resonant with what Q in a selected resonator.